The present application is directed to a main rotor blade tip design for rotary-wing aircraft and more particularly to a main rotor blade tip planform that enhances hover performance by the combined tailoring of blade tip sweep, blade tip taper and blade tip anhedral.
In general, airfoils have a limited range of Mach numbers and angles of incidence with respect to the relative wind over which they operate efficiently. In general, it may be claimed that the efficiency of a hovering rotor will continue to improve as rotor thrust, and simultaneously, airfoil lift continues to increase. However, once the tip airfoil maximum lift coefficient is approached at its operating Mach number, the drag of the tip airfoil increases rapidly, and the hovering efficiency of the rotor system accordingly drops. Typically, an airfoil operating at a higher Mach number will stall sooner than one operating at lower Mach numbers. Because the tip of the hovering rotor blade is operating at the highest Mach number, it will be the first to experience the onset of stall and, therefore, an increase in drag, resulting in a drop off in rotor hovering efficiency.
The maximum lift of a rotor blade in hover can be extended and higher hover efficiencies reached by improving the maximum lift coefficient of the tip airfoil at hover critical Mach numbers typically in the rage of approximately 0.5 to approximately 0.6. However, the use of high lift airfoils over the tip region of rotor blades is of minimal use if the rotor blade forward flight performance efficiency becomes resultantly restricted as a consequence of advancing and retreating side airfoil requirements that differ from hover requirements.
For conventional rotor blade designs, the maximum lift that can be efficiently obtained in hover is already relatively well matched with the gross weight that can be carried efficiently in forward flight. That is, the hover efficiency (Figure of Merit) is optimized at an aircraft maximum gross weight which is already generally equivalent to the aircraft maximum gross weight during forward flight operations.
Increasing operational lift levels in both hover and forward flight may be achieved with improved airfoil geometry design alone. To do so, the airfoil must increase lift at hover critical tip Mach numbers of 0.5 and 0.6, as well as provide an increase in airfoil lift at the forward flight retreating side critical Mach numbers of 0.3 and 0.4. Simultaneously, adverse Mach related drag divergence impact must be avoided as the airfoil experiences Mach numbers above 0.8 on the advancing side of the rotor disk. While current computer based airfoil design codes are capable of assisting in the development of optimized blade designs, the design of a multitude of airfoils distributed along the rotor blade span that meet the required lift, drag and pitching moment operational and performance criteria in hover and forward flight is very difficult.
The approach to proper rotor blade tip design is to carefully combine various blade tip geometric attributes and airfoils to provide the best hover and forward flight performance gains to be realized. In other words, performance shortfalls that may occur in forward flight as a result of using hover optimized airfoils for improved hover efficiency may be resolved by carefully altering other blade geometric parameters so as to simultaneously improve forward flight efficiency.